1. Field of the Invention
The present invention relates to a second air flow control apparatus, and more particularly to a second air flow control apparatus that is disposed in a second air supply path at an engine exhaust port for controlling a second air flow into the engine exhaust port.
2. Description of related art
Catalytic converter is a metal cylinder with catalytic coating that contains rare metal such as Pd and Pt. The catalytic coating can convert the harmful CO and HC in exhaust gas into harmless CO2 and H2O, or convert the harmful NOx in the exhaust gas into harmless N2 and O2, so as to clean the exhaust gas and alleviate the negative effect of exhaust gas on the environment. For facilitate the oxidation reactions between the catalyst and the harmful substance in the exhaust gas, second air has been flown to an engine exhaust port. In this technology, a second air supply tube is disposed on the engine exhaust tube before the catalytic converter. The pressure of the exhaust gas pushes the fresh air outside, i.e., second air, to flow into the gas exhaust tube so as to increase the amount of O2 in the exhaust gas and improve the efficiency of the oxidation of CO and HC by the catalyst.
It is known that when automobiles travel at a speed of 35 to 55 km/hr in a crowded city, the CO amount in the engine exhaust gas is relatively small (shown in FIG. 25), but the NOx amount in the engine exhaust gas is large (shown in FIG. 26). Now, if second air is continuously provided into the engine exhaust tube, it is not good for the catalyst to reduce the NOx in the exhaust gas. So in a conventional technology, a pressure control valve connected to the engine gas inlet or engine outlet is used so as to control the second air flow in the engine exhaust port (gas exhaust tube) by the gas pressure of the engine.
However, the pressure control valve is not capable of precisely controlling the second air flow in the engine exhaust port. The catalyst still can not effectively reduce the NOx so that some of the NOx is emitted to the external air. Such NOx is a primary cause for acid rains and damages on the ozone layer and has a very negative impact on the natural environment. Hence the above-mentioned problem needs to be solved.